1. Field of the Invention
This invention relates to methods and apparatus for running casing into a wellbore, and more particularly, to methods and apparatus for creating a buoyant casing chamber in the casing to lighten the casing so that it may be run to a greater depth in the well.
2. Description of the Prior Art
In many wells, particularly horizontal or highly deviated wells, it is often difficult, if not impossible, to run well casing to the drilled depth of the well due to high casing drag usually caused by hole geometry, casing size, hole size, excess cutting in the hole. Creating a downhole buoyant chamber in the casing lightens it and increases the likelihood of success in getting casing to the bottom of the drilled hole. That is, if the string of casing can be made lighter, friction drag is reduced, and obstacles are more easily overcome.
U.S. Pat. Nos. 4,986,361; 5,117,915; and 5,181,571 disclose well casing flotation devices and methods of use. All of these patents are owned by Union Oil Company of California (UNOCAL). These patents relate to the creation of a buoyant casing chamber before the casing is run into the wellbore. The chamber cannot be created once the casing is run to its full depth. Also, the apparatus in these patents require that the operator determine the length of the air chamber prior to running the casing. Once the casing has been run into the wellbore, the length of the buoyant chamber cannot be changed.
The present invention solves this problem by providing, in some embodiments of the invention, for the creation of a buoyant casing chamber after the casing has been run a significant depth into the well. In this way, the length of the buoyant casing chamber can be determined based on downhole well conditions which might not be readily determined before the casing is run. This allows greater flexibility for the operator, and even avoids the necessity of creating a buoyant chamber if the casing can be run to the bottom of the well initially. Obviously, if the casing can be run to the bottom of the well, there is no need to incur the cost or take the time necessary to create a buoyant chamber.
When running the casing into the well, it is very desirable to have the ability to circulate fluid as the casing is being run in order to wash the casing past ledges and bridges often encountered, as well as providing lubrication for the casing to minimize drag on the wellbore. Also, it is often necessary to wash wellbore cuttings from horizontal and highly deviated sections of wellbores to allow passage of the casing. It may be further necessary to circulate large amounts of well cuttings out of the hole to allow passage of the casing.
U.S. Pat. Nos. 5,117,915 and 5,181,571, mentioned above, show an apparatus which allows circulation during the running-in of the casing. The present invention also provides different embodiments where fluids may be circulated while still providing a casing buoyant chamber.
The present invention provides for methods and apparatus for creating a downhole buoyant casing chamber. Each of the embodiments provides that the buoyant casing chamber may be created after the casing has been at least partially run into the wellbore. Certain of the embodiments also allow circulation of fluid as the casing is being run after the creation of the buoyant chamber.
Generally, the present invention includes a method of installing casing in a well in which the method comprises the steps of running a length of casing into the well to a first depth, forming a buoyant chamber in the casing, filling the chamber with a buoyant fluid, and running the casing to a second depth greater than the first depth. The buoyant fluid may be a gas or a liquid with a lower specific gravity than the well fluid.
In a first embodiment, the step of forming a buoyant chamber comprises sealing a lower end of the casing, providing a passageway inside the casing through which the buoyant fluid may be injected, and sealing between the casing and the passageway above the lower end of the casing. The casing preferably has a floating device adjacent to a lower end thereof, and the step of forming the buoyant chamber further comprises positioning a packer, with a subsurface release plug on a lower end thereof, in the casing above the floating device, actuating the packer into sealing engagement with the casing, releasing the plug from the packer, and injecting the buoyant fluid into the casing, thereby moving the plug downwardly into engagement with the floating device.
In another embodiment, the casing also has a floating device adjacent to a lower end thereof, and the step of forming the buoyant chamber comprises positioning tubing in the casing above the floating device, the tubing having a subsurface release plug on a lower end thereof and having a packer thereon above the plug, actuating the packer into sealing engagement with the casing, injecting the buoyant fluid above the plug, thereby moving the plug and tubing downwardly such that the plug is placed into engagement with the floating device. In running the casing to the second depth, fluid may be circulated through the tubing.
In a third embodiment, the first depth corresponds to a length of the buoyant chamber, and the casing has a floating device therein. In this embodiment, the step of forming the buoyant chamber comprises positioning tubing in the casing above the floating device, the tubing having a stinger on a lower end thereof and a packer above the stinger, actuating the packer into sealing engagement with the casing, thereby trapping the buoyant fluid in the buoyant chamber between the casing and tubing, and circulating fluid through the tubing.
The present invention may also be said to include a method of installing casing in a well comprising the steps of forming a buoyant chamber in a length of casing, running the casing into the well to a desired depth, and circulating fluid through the casing while running the casing into the wellbore.
The apparatus of the present invention generally comprises a length of casing, a floating device disposed in a lower end of the casing and forming a lower boundary of a buoyant chamber, sealing means for sealingly engaging the casing at an upper end of the buoyant chamber, and a volume of buoyant fluid to fill the buoyant chamber.